In Patent Literature 1, a wastewater treatment system in which organic wastewater is treated by an anaerobic treatment is described. The system includes an acid production tank (acid fermentation tank) in which raw wastewater is subjected to an acid fermentation treatment, and a reaction tank (methane fermentation tank) in which the wastewater subjected to the acid fermentation treatment is subjected to pH adjustment by a neutralizer and is then subjected to an anaerobic treatment. By supplying biogas produced in the methane fermentation tank to the acid fermentation tank, the concentration of ammonia, hydrogen sulfide, and hydrogen in the acid fermentation tank can be decreased by a stripping effect of the biogas.